Importance: It is unknown whether state recreational cannabis legalization (RCL) is related to increased rates of prenatal cannabis use or whether RCL-related changes vary with cannabis screening methods or the local policy environment.

Question: Did rates of cannabis use during early pregnancy change after recreational cannabis legalization (RCL) in California, and did trends differ in jurisdictions that allowed vs banned adult-use retailers?

Findings: This time-series study of 300 993 pregnancies universally screened for prenatal cannabis use at entrance to prenatal care in Northern California from 2012 to 2019, there was an increase in self-reported and biochemically verified prenatal cannabis use after RCL implementation in 2018. The increase was observed only among those living in jurisdictions allowing adult-use cannabis retailers.

Meaning: The implementation of RCL in California was associated with an increase in prenatal cannabis use, driven by individuals living in jurisdictions that permitted adult-use retailers.